A conventional driving device for a synchronous motor includes an inverter circuit, which is comprised of six transistors and six diodes, and a control circuit that controls the inverter circuit as disclosed in US 2002/0070715 A1 (JP 2002-10654 A).
The inverter circuit is formed by connecting three pairs of series-connected transistor in parallel between a positive pole bus and a negative pole bus. In addition, one diode is connected in parallel with each transistor.
A direct current power supply is connected between the neutral point of the stator coil and the negative pole bus of the inverter circuit. A capacitor is connected between the neutral point of the stator coil and the positive pole bus of the inverter circuit. For this reason, a voltage difference equivalent to the sum of the supply voltage produced at the direct current power supply and the voltage produced at the capacitor is produced between the positive pole bus and the negative pole bus.
The control circuit causes the six transistors to perform switching operation and thereby outputs three-phase alternating currents to a three-phase alternating current synchronous motor based on the voltage difference between the positive pole bus and the negative pole bus. Therefore, the three-phase alternating current synchronous motor can be operated with a voltage higher than the supply voltage produced by the direct current power supply.
When, of the six transistors, the transistors connected to the negative pole bus are turned on, a current is passed through the stator coil. Therefore, magnetic energy is stored in the stator coil based on the current.
When the transistors connected to the negative pole bus are turned off, a current based on the above magnetic energy flows from the stator coil to the capacitor by way of the diodes on the positive pole bus side and the positive pole bus. Therefore, three-phase alternating currents are passed through the stator coil and electric charge is stored in the capacitor by causing the six transistors to perform switching operation.
In the above driving device, when control of the inverter circuit is started under the condition that electric charge has not been stored in the capacitor, a large rush current flows from the stator coil to the capacitor by way of the diodes on the positive pole bus side and the positive pole bus at the time when the transistors connected to the negative pole bus transition from on to off. In conjunction therewith, the output voltage of the capacitor largely fluctuates.
Therefore, the alternating currents outputted from the inverter circuit to the three-phase alternating current synchronous motor becomes unstable. This will cause vibration in the three-phase alternating current synchronous motor.